This invention relates to a vinylidene fluoride resin fiber improved in tensile strength and a process for producing the same.
Vinylidene fluoride resin fibers, due to excellent characteristics of the base resin such as weathering resistance, oil resistance, water resistance etc., are potentially suitable for a wide scope of uses requiring such characteristics, for example materials for industrial uses including ropes for industrial application, fabrics, other construction materials, materials for transportation, etc., or materials for leisure use such as fishing lines, strings for musical instruments, etc. However, the problem encountered in applying the vinylidene fluoride resin fiber for such uses as mentioned above has been its low tensile strength.
The tensile strength, for example, in ropes for industrial application, is a factor which determines how slender a rope can sustain a predetermined load, or in fabrics, is a factor which determines basically the mechanical strength, typically durability against hooking, etc.
For this reason, for vinylidene fluoride resin fibers, similarly to other resin fibers, attempts have been made to improve their tensile strength, but satisfactory results have not necessarily been obtained. For example, the basic method for improvement of tensile strength conventionally attempted for the vinylidene fluoride resin fiber has been one aiming at increasing the degree of orientation as large as possible. However, according to this method alone, even if the orientation degree may be made larger, a tensile strength of at most 80-90 kg/mm.sup.2 can only be obtained. There is also an attempt to apply to vinylidene fluoride resins the ultradrawing method which is effective in obtaining high strength fibers from polyethylene or polypropylene, namely the method in which cold stretching is performed at a very slow speed to a large stretching degree of 30 to 35-times. Although this method may be successfully applied to polyethylene or polypropylene which has a small intermolecular cohesive force, no good fiber product has yet been obtained from a vinylidene fluoride resin which has a large intermolecular cohesive force. On the other hand, a high strength is obtained by spinning from a dope in liquid crystal-state of a totally aromatic polyamide resin having very rigid polymeric chains. But, it is impossible in principle to apply such a liquid crystal-state spinning method to vinylidene fluoride resins. This is because vinylidene fluoride resins are so-called flexible polymers comprising carbon-carbon single bonds, and therefore they cannot take a liquid crystal-state in a solution. Accordinly, even when spun from a solution state, they can not take a liquid crystal-state, and thus fail to give a fiber with a high strength.